Constraining the Age of Quaternary Megafloods in the Altai Mountains (russia) Using Luminescence

Some of the largest catastrophic outbursts of periglacial lakes known in the geological history of the Earth have been identified in the Altai Mountains. Traces of these events are recorded in the form of large terraces, pre-dominantly composed of gravel material with numerous horizons of large boulders and blocks. Determining the age of these large-scale events is difficult due to the lack of suitable material (e.g. organics, well-bleached sand) and the specific genesis of these sediments. The results of cosmogenic radionuclide dating suggest a post-LGM age both for the source of the flood water and for different elements of the catafluvial terraces in the Chuya and Katun river valleys. Nevertheless, the age(s) of catastrophic breakthrough remains controversial. On the basis of a few IRSL ages, and geological and other evidence, some view the event as occurring around MIS 5. In this study, we investigate loess-like loams overlying the catafluvial sediments on the surface of the highest level terrace,-200 m above present river level. A total of 24 samples for luminescence dating were obtained, for which the OSL, IR50, and pIRIR50,290 signals were measured to control the degree of signal zeroing and the dating reliability. The age of the loess in all three pits was from 0.5 ka at the top to 23 ka at the base of the loess strata. From a sand layer in the top of the catafluvial deposits, two ages of-85-90 ka were obtained from feldspar pIRIR50,290. These results provide a minimum pre-LGM age for the geomorphological surface of a major cata-fluvial terrace in the Altai Mountains.